Parse json inside another json string in swift4

1. The string for key feed is not valid JSON. You have to replace the single quotes with double quotes.
2. Create a Data object from the string (casting the type doesn't work).
3. Create a JSON object from the data.
4. Get the values you need.

Side note:

Don't use NS... collection types and NSString in Swift.

Use Codable to parse the above JSON response.

Models:

struct Root: Codable {
    let count: Int
    let next: String
    let previous: String?
    let results: [Result]
}

struct Result: Codable {
    let id: Int
    let feed, createdAt: String
    let sensor: Int
}

Parse the JSON data like so,

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let response = try decoder.decode(Root.self, from: data)
    print(response)
} catch {
    print(error)
}

First, peel off the piece you want:

struct ResultWrapper: Decodable {
    let results: [Result]
}

And then build a custom decoder to extract feed, which is malformed. Single-quotes are not legal JSON. As a hack, the following code just substitutes all single-quotes with double-quotes, but this won't work if there are any embedded quoted-single-quotes.

struct Result: Decodable {
    enum CodingKeys: String, CodingKey { case feed }
    let feed: Feed
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        // inner JSON is mal-formed. Have to fix it.
        let feedString = try container.decode(String.self, forKey: .feed)
            .replacingOccurrences(of: "'", with: "\"")

        feed = try JSONDecoder().decode(Feed.self, from: Data(feedString.utf8))
    }
}

Finally, decoding Feed is mechanical, but requires custom CodingKeys. I recommend quicktype for that:

// https://app.quicktype.io?share=7O6iNJ3ugXb4mr84TIoG
struct Feed: Codable {
    var appID, devID, hardwareSerial: String
    var port, counter: Int
    var payloadRaw: String
    var payloadFields: PayloadFields
    var metadata: Metadata

    enum CodingKeys: String, CodingKey {
        case appID = "app_id"
        case devID = "dev_id"
        case hardwareSerial = "hardware_serial"
        case port, counter
        case payloadRaw = "payload_raw"
        case payloadFields = "payload_fields"
        case metadata
    }
}

// MARK: - Metadata
struct Metadata: Codable {
    var time: String
    var frequency: Double
    var modulation, dataRate: String
    var airtime: Int
    var codingRate: String
    var gateways: [Gateway]

    enum CodingKeys: String, CodingKey {
        case time, frequency, modulation
        case dataRate = "data_rate"
        case airtime
        case codingRate = "coding_rate"
        case gateways
    }
}

// MARK: - Gateway
struct Gateway: Codable {
    var gtwID: String
    var timestamp: Int
    var time: String
    var channel, rssi: Int
    var snr: Double
    var rfChain: Int
    var latitude, longitude: Double
    var altitude: Int
    var locationSource: String

    enum CodingKeys: String, CodingKey {
        case gtwID = "gtw_id"
        case timestamp, time, channel, rssi, snr
        case rfChain = "rf_chain"
        case latitude, longitude, altitude
        case locationSource = "location_source"
    }
}

// MARK: - PayloadFields
struct PayloadFields: Codable {
    var aamsgType: String
    var abstatus: Int
    var batteryV: Double
    var batteryLow: String
    var humiP, tempC: Double

    enum CodingKeys: String, CodingKey {
        case aamsgType = "aamsg_type"
        case abstatus, batteryV
        case batteryLow = "battery_low"
        case humiP, tempC
    }
}